Corrosion resistant lubricating grease compositions



CORROSION RESISTANT LUBRICATING GREASE COMPOSITIONS Joshua D. Srnith and Harris L. Hendricks, New Orleans, La., assignors to Shell Development Company, New York, N. Y., a corporation of Delaware No Drawing. Application May 17, 1955 Serial No. 509,1 9 Claims. (Cl. 252-333) This invention relates to lubricating grease compositions. More particularly, it is concerned with grease compositions containing certain classes of water-soluble corrosion inhibitors, calcium mahogany sulfonates, and a gel-forming lithium soap of hydroxy fatty acids.

Grease compositions broadly comprise a lubricating oil gelled to a grease consistency with a gelling agent which may be an inorganic colloid or, more commonly, relatively high molecular weight metallic soaps. Under the conditions where greases are employed, these compositions are continuously or intermittently subjected to humid or wet atmospheres which provide an aqueous environment which, even in relatively small amounts, permits the corrosion of metallic surfaces being lubricated. Several means of avoiding or minimizing these corrosion phenomena have been investigated, such as the use of corrosion resistant alloys, the coating of metal parts (such as metal bearings) with relatively permanent plastic or metallic coatings, the sealing of bearings to prevent the entry of water vapor, and the incorporation of corrosion inhibiting materials in the grease compositions themselves.

Corrosion inhibitors are materials well known in the lubricating art and in allied arts wherein metallic equipment must be protected from destructive results of corrosive conditions. Corrosion inhibitors are known to perform their function by several diflerent mechanisms, such as the formation of monomolecular films either by reaction with or adsorption on the metallic surfaces. The corrosion inhibitor may be oil-soluble or water-soluble, but since corrosion appears to proceed with greatest rapidity in an aqueous medium, the most effective corrosion inhibitors for most situations are those possessing a substantial amount of water-solubility. The latter state is satisfactory, however, only when the greases are utilized for the protection of metallic surfaces under static or quiescent conditions. When the grease film is being constantly sheared and displaced (by turning of a bearing, for example), it has been ascertained that complete corrosion protection is not obtained. Furthermore, the presence of water-soluble soaps and similar substances as well as the presence of water-soluble corrosion inhibitors has two deleterious effects upon the grease composition: In the first place, the presence of these water-sensitive materials increases the water sensitivity of the grease composition so that it is more readily emulsified and washed out of bearings under wet conditions. Moreover, for some reason as yet unexplained, it has been found that the presence of such materials has a degrading effect upon the grease yield so that for a given grease consistency an increased amount of gel-forming soap is required.

Also due to the fact that the water-soluble corrosion inhibitors are not fully miscible with the oleaginous fluid normally included as the lubricating component of the grease, it is often diflicult to maintain a satisfactory dispersion of the corrosion inhibitor during the entire life of the grease. This is especially apparent under quiescent conditions and in storage, when the water-soluble corrosion inhibitors tend to settle to the bottom of a storage container. Efforts have been made to correct this disadvantage by the incorporation together with the Watersoluble corrosion inhibitor of a water-soluble greaseforming soap such as sodium hydroxy stearate or sodium stearate. While this does, in fact, promote the better dispersal of the corrosion inhibitor throughout the grease composition, it simultaneously softens the grease composition for some unknown reason as was referred to hereinbefore and consequently requires a substantial increase in the total soap content to reach a given grease penetration.

The problem of non-homogeneity of the grease is not only restricted to a settling of the corrosion inhibitors to the bottom of a storage container but alsois evidenced in an unsatisfactory property referred to as graininess. By this is meant the phenomenon of crystal growth of the corrosion inhibitor in the grease composition to such an extent that the crystals are screened out of the grease when the latter is passed through a homogenizing screen toward the end of its manufacturing process. This results in a clogging of the screens and, more importantly, removal of the corrosion inhibitor from the grease composition, leaving the grease deficient in corrosion protective properties.

A particularly serious problem is encountered when utilizing lithium soaps of hydroxy fatty acids as the principal gelling agent for a grease composition due to the peculiar property of such soaps in being deficient with respect to afi'inity for metallic surfaces which they are supposed to be lubricating and protecting from corrosion. This would appear to be unique, insofar as the soaps are concerned, since other soaps regardless of their wateror oil-solubility have been found to adhere with greater or less degree to metallic (particularly ferruginous) surfaces such as bearings and the like. Consequently, while the lithium hydroxy fatty acid soap greases are favored for their excellent properties in other respects, they have been found to permit an undue amount of corrosion due to their frequent complete loss of affinity for metal surfaces when the grease has adsorbed water to the saturation point even though the grease may still maintain a good consistency. This lack of aflinity for metal prevents the grease under wet operating conditions from adhering to the metallic surface and not protecting them from corrosion. It also allows the grease to be easily squeezed or worked out of the bearings leaving them dry of grease with subsequent failure due to the lack of lubrication. This is especially evident under oscillating type of motion, but it also is a major cause of corrosion under static conditions as well. The lack of aflinity of lithium hydroxy fatty acid soap greases for metal allows a film of water to form between the grease and the metallic surface and, unless steps are taken to counteract this, results in severe corrosion of the bearing surface.

It is an object of the present invention to provide improved grease compositions. It is another object of this invention to provide grease compositions having improved corrosion properties. It is a particular object of this invention to provide grease compositions wherein water-soluble corrosion inhibitors present therein are permanently dispersed throughout the grease composition. It is an additional object of this invention to provide a means of improving grease compositions wherein the water-soluble corrosion inhibitors present therein do not separate in the form of crystals large enough to be screened out of the composition during homogenizing screening operations. Still another object of the present invention is to improve the above described properties of the grease without deleterious effects to the consistency thereof. Other objects will become apparent during the following discussion.

Patented Jan. 14, 1958 asadooo New, in accordance w-ith the present invention, it has been found that water-soluble nitrites and/or chromates may be eifectively and permanently dispersed in greases gelled with lithium soaps of hydroxy fatty acids by also incorporating in" said greases an amount less than about 5% by weight,- based on the total grease composition, of calcium salts ofmahogany sulfonic acids. preferably, the nitrit'e's (or chromates) are present in the grease composition in the form of an aqueous solution, the proportion of Water being between about 0.5 and 2.5% by weight ofthe total grease.

More specifically, it has been found that the presence of the calcium mahogany sulfonates not only prevents the separation ofthe nitrite (or chromate) from the grease composition, but also does not cause any softening of the grease such as is occasioned by the use of watersoluble sodium soaps, such as sodium stearate and the like.

Still further in accordance with the present invention,

it' has been found that the presence of the calcium mahogany sulfonates has the surprising eifect of greatly improving the metal affinity of the grease and thus substantially improves the protective properties of the grease under both static and dynamic conditions. The improvement in the grease with respect to metal afiinity results insubstantially longer bearing life under dynamic conditions, especially as measured with an oscillating friction machine to the described hereinafter.

The Water-soluble nitrites or chromates to be utilized in accordance with the present invention are well known for their corrosion inhibiting properties. They comprise especially the alkali metal nitrites or chromates, but may include the amine and ammonium nitrites and chromates as well. Typical species of these materials include especially sodium nitrite, potassium nitrite, lithium nitrite, sodium chromate, potassium chromate, ammonium nitrite and ammonium chromate, as well as alkyl ammonium nit-rites. such as diisopropyl ammonium nitrite and cycloaliphatic amine nitrites, typical of which is dicyclohexylamine nitrite. These are to be used in the grease compositions in amounts between about 0.1 and about 1% by weight of the total soap. Preferably they are present in the form of aqueous solutions of such concentration that the water is present in amounts between about 0.5 and about 2.5%, based on the total grease weight. However, they may be utilized in substantially anhydrous greases .as well. The presence of the nitrite or chromate in the form of an aqueous solution further insures the maintenance of a water-soluble material in non-crystalline form which is most readily available for mixture with any contaminating water, thus rendering the latter non-corrosive to metals with which it comes in contact. The problems solved by the present invention comprises the permanent dispersal of this aqueous solution of the water-soluble corrosion inhibitor in such a manner that the consistency of the grease is not degraded. This is efiected by the presence of the calcium mahogany sulfO l tes.

The calcium hydrocarbyl sulfonates are preferentially oil-soluble, preferably obtained from petroleum sources, and are known as petroleum sulfonates. These are ordinarily obtained in the treatment of petroleum oils to obtain highly refined products of the type of electrical insulating oils, turbine oils and the like, in which the petroleum oils are treated successively with a number of portions of concentrated sulfuric acid or fuming sulfuric acid. A variety of compounds are formed by the chemical reactions of sulfuric acid upon the oil including sulfonic acid, organic esters of sulfuric acid, partial esters of sulfuric acid, etc. Most of these compounds are relatively insoluble in the oil under the treating conditions and separate from the oil together with unreacted sulfuric acid as a sludge, which is separated from the oil after each treatment. The sulfuric acid is usually added in dumps" of about /2 lb. per gallon of the oil, the- More total quantity of the acid added depending upon the oil being treated and the desired final product. Usually from about 3 pounds to about 9 pounds of sulfuric acid per gallon of oil are used. Some of the sulfonic acids resulting from the treatment of the oil with the sulfuric acid are preferentially oil-soluble and. remain in the oil layer after removal of the acid sludge. These can be removed from the oil by neutralizing the acid-treated oil with an alkaline agent such as oxides, hydroxide, or carbonate of a calcium, for example, calcium oxide, to form sulfonic acid soaps or sulfonates which are then extracted from the oil by treatment with 50% to. aqueous alcohol solutions or other suitable means. Because of the characteristic mahogany color of the sulfonates they are known in the petroleum art as mahogany soaps. While the majority proportions of the preferentially oil-soluble sulfonates are obtained from the acid-treated oil, there can be recovered from the acid sludge by suitable solvents preferentially oil-soluble sulfonates or sulfonic acids. The term preferentially oil-soluble sulfonates therefore includes the oil-soluble sulfonates from both the acid-treated oil and the acid sludge. referably the sulfonic acids have molecular weights in the order of about 400 to 750.

The calcium salts or soaps of the subiect sulfonic acids may be either the so-called neutral soaps wherein all of the hydroxyl groups of the neutralizing agent such as lime are combined with sulfonic acid radicals, or the so-called basic sulfonates may be employed wherein the metallic radical has one valence attached to a sulfonic acid radical and the other being a hydroxyl substituent.

The preferred sulfonates are the so-called basic sulfonates having the simplified general formula represented by (RSOQM-OH wherein R is a hydrocarbyl radical, M is a calcium radical, R usually being a naphthenyl, alkyl naphthenyl, alkaryl or naphthenyl aryl radical, such that the weight of the sulfonic acid is that stipulated hereinbefore.

The calcium mahogany sulfonates are utilized in amounts between about 0.5% and about 5% by weight of the total grease composition and preferably in amounts between about 1% and about 2.5% by weight thereof.

The lithium soaps utilized as the principal gelling agents for the present invention comprise the soaps of hydroxy fatty acids having between about 12 and about 24 carbon atoms per molecule. These are typified by the acids derived by saponification of hydrogenated castor oil acids which predominate in ricinoleic acid. The glycerides, methyl esters or the free acids may be employed in the saponification procedure. The soap may be formed in situ or may be pro-formed prior to incorporation together with the other grease-forming ingredients. Other suitable hydroxy fatty acids include 9,10-dihydroxy stearic acid, 4-hydroxy palmitic acid, quince oil acids, and the like, however, the hydrogenated castor oil acids are preferred.

The mineral lubricating oil to be employed as the major ingredient of the present grease compositions may be of any suitable lubricating viscosity, ranging from about 50 SSU' at F. to about 2000 SSU at 100 F. The viscosity index of the oil can vary from below 0 to about 100 or higher and the oil can have averagev molecular weights ranging from about 250 to about 800. It may be highly refined and solvent treated if desired by known means. The hydrocarbon oil may be of synthetic or mineral origin, although mineral oils are preferred.

While low viscosity index and low viscosity oils may be utilized, it is preferred for various reasons to employ oils having viscosity indices above about 70 and preferably Within the range of about 75 to about 95. The viscosity of the oil is preferably about 70 SSU at 210 F. Preferably oils derived from Bright Stock, e. g. residual lubrieating oils, comprise at least one-half of the lubricating oil components.

assume Suitable" rease compositions: having pro erties asscrib'ed hereinbe'foreand the above described combination of components comprise the following ifigr'edieti'ts in the stated proportions:

Lithium hydroxy fatty acid soaps (ge1- ling proportion) Mineral oil constituting the balance of the composition with the exception of other minor optional ingredients Usually 5'-25%.

such as: V

Water 01-25% (preferably 05-23%). Oxidation inhibitors; O.12.5,0.

And other welHrnowlI greaseadditives.

In exarniningthe ability of-a greaseto adhere to metal under wet coiiditions the following test was employed using the Shell roll stability test apparatus: In this apparatus 100 grams of grease were placed in the cylinder together with cc. of water containing 0.05%sodiur'n chloride. The steel roller wasinserted and the apparatus closed and rolled at a constant speed and at room temper'-' ature for one-half hour. At that time it was stopped and opened to determine whether or not the-grease still clung to the interior surfaces, and if water had been absorbed into the grease. The latter was tested for consistency by the micropenetration test. Each one-half hour succeeding, 10 cc. portions of the salt water were added and the above cycle was repeated, allowing minutes rolling and 10 minutes for the described inspection tests. The tests were terminated when one of three criteria was reached; first, the grease failed to stick to the interior surfaces, i. e. it lost its metal aflinity. When this occurred the grease would separate from the inside Wall of the cylinder, the roll surface and the cylinder ends, leaving them completely clean with no grease whatsoever adhering to the surfaces. The test also was terminated when the grease would absorb no more water. At this point the water would remain separate from the grease even after the standard rolling period. The third possible means of terminating the test was at the point at which the grease became too soft for further use. This point was arbitrarily chosen at a micro-penetration of 230 drnm.

Under these conditions, the standard well known commercial greases were tested having a variety of soaps as the gelling agent and all having mineral lubricating oils as the lubricating component. The soaps were present in each case in an amount to provide a medium consistency grease. Thus, greases containing sodium stearate, calcium stearate, lithium IZ-hydroxy stearate, aluminum stearate, lithium stearate, mixtures of calcium and lithium hydroxy stearates, mixtures of lithium and sodium hydroXy stearates, were tested.

When between 40 and of the salt Water solution had been added to lithium IZ-hydroxy stearate grease, the latter failed to adhere to the metal surfaces and the test was terminated. None or" the other greases tested lost their metal afiinity at this low a water percentage, and in a number of instances, the tests with these other greases were continued to water concentrations in the order of 140% by weight of the grease and more. Consequently, it will be seen from these results that the lithium hydroxy stearate greases are unique with respect to their limited metallic afiinity the presence of relatively minor amounts of water.

The addition of about 0.5% by weight of sodium nitrite to the same lithium l2-hydroxy stearate grease did not improve the metal aflinity characteristics thereof. The combination of sodium stearate soap in an amount of about 0.4% by weight of the grease together with 1% of sodium nitrite did not alter the poor metal adhesion characteristics of the grease. The sodium stearate was added as a dispersant tor the sodium nitrite. While it was eiiective for this purpose to a certain extent, it did not improve the metal adhesion characteristics of the total grease composition. However, upon the addition of 2:59? by weights of eaiiam mahogany Sui-rem ssion? lithium 12 h'yd'roXy' stear'ate grease c0i1taining 0;5% by weight ofsodium nitrite, the metal afiin'ityfof"th'ef'grease substantially improved so' that even at saltwater" content the grease still adhered to the Wall sottlieShell Roll Tester.

The static corrosion protection of the subjecftype'of greases was determined by'th'e' glassfjarco'rrosion' 'fest?" which consists in smearing a metallic panel'with'the" grease and immersing the coated panel for 1'20hours" at room temperature in water'conta'ining 0.05% sodium chloride. The base grease, namely, a" gellingamou'n't of lithiurnl-Z-hydmxy stearate in mineral lubricatingioilcontaining'no inhibitors, permittedhea-vy' corrosion tdo'c'curj' under these test conditions. Thein'corpora'tion of 10.5% or of 1% sodium nitrite in the base" grease resultedfin slight corrosion of the panel after the test period. The incorporation of 1.5% calcium maltogany's ulfoiiate's' in addition to either amount of sodiurn'ni'trite resulted in complete corrosion protection under these conditions;

Apparently due to the beneficial effect of the calcium mahogany sulfonates uponthe metal aflinityof tli'elithiu'fn" hydroxy stearate greases containing sodium nitrite, the corrosion characteristics of the grease underdynamic operating conditions were surprisingly improved. This property was tested by the oscillating friction machine test which is performed as follows? A bearing. operas big man oscillating manner was coated with a rease" and continually drenched with water in the oscillating friction machine for a period of 2 hours at ambient ternperature followed by a dry operating period, the cycle being repeated until a sharp temperature rise occurred. Under these conditions it was found that the lithium l2-hydroxy stearate grease permitted operation for a period of about 75 hours. The incorporation of 0.5 or of 1% sodium nitrite in the same grease had no beneficial effect upon the time of operation under these test conditions. However, upon addition of about 1.5% by weight of calcium mahogany sulfonates the test life of the bearing was substantially doubled to 156 hours.

The base grease utilized in the above comparative testing procedures comprised about 8% by Weight of lithium 12-hydroxy stearate, and about 87-91% by weight of mineral lubricating oil. To this was added 0.5 by weight of sodium nitrite contained in about 0.7% by weight of water. 1.5 by weight of calcium mahogany sulfonates was employed where indicated. The lubricating oil has a viscosity index of 65.6, and a viscosity of 485 SSU at F, and 57.4 SSU at 210 F. The greases were prepared by saponification in the presence of a portion of the oil after which the balance of the oil was added and the mixture heated to 390 F. with circulation through a homogenizing screen. After the mixture had been dehydrated, it was cooled to about 200 F circulation being continued, While the additives (where indicated) were incorporated. The calcium mahogany sulfonates were the so-called basic sul'fonates.

We claim as our invention:

1. A lubricating composition comprising a mineral lubricating oil as the predominant constituent, the oil being thickened to a grease consistency with lithium 12- ltydroxy steal-ate, said grease also containing about 1.5% calcium mahogany sulfonates, 0.5 sodium nitrite, 0.6% water and 1% phenyl-tt-naphthylamine.

2. A lubricating composition comprising a mineral lubricating oil as the predominant constituent, at least about one-half of said oil being a mineral oil Bright Stock, the oil being thickened to a grease consistency with lithium soaps of hydrogenated castor oil acids, 0.55% of calcium salts of mahogany sulfonic acids, 0.1l% by weight of an alkali metal nitrite and 0.52.5% by weight of water.

3. A lubricating grease comprising a mineral lubricating oil as the predominant constituent thickened to a grease consistency with about 525% by weight of lithium 7 soaps of soap-forming substantially saturated hydroxy fatty acids, about 0.55% by weight of calcium salts of mahogany sulfonic acids and 0.1-l% by weight of an alkali metal nitrite and 0.5-2.5% by weight of water- 4. A lubricating grease comprising a mineral lubricating oil as the predominant constituent thickened to a grease consistency with about 525% by weight of lithium IZ-hydroxy stearate, 0.55% by weight of a calcium salt of mahogany sulfonic acids, and about (Ll-1% by weight of an alkali metal nitrite.

5. A lubricating grease comprising a mineral lubricating oil as the predominant constituent thickened to a grease consistency with about 5-25 by weight of lithium soaps of soap-forming substantially saturated hydroxy fatty acids, about 0.5-5% by weight of calcium salts of mahogany sulfonic acids and 0.1-1% by weight of sodium nitrite.

6. A lubricating grease comprising a mineral lubricating oil as the predominant constituent thickened to a grease consistency with about 5-25 by weight of lithium soaps of soap-forming substantially saturated hydroxy fatty acids, about 0.5% by weight of calcium salts of mahogany sulfonic acids and 0.1-1% by weight of an alkali metal chromate.

7. A lubricating grease comprising a mineral lubricating oil as the predominant constituent thickened to a grease consistency with about 525% by Weight of lithium soaps of soap-forming substantially saturated hydroxy '8 fatty acids, about 0.55% by Weight of calcium salts of mahogany sulfonic acids and (ll-1% by weight of an alkali metal nitrite.

8. A lubricating grease comprising a mineral lubricating oil as the predominant constituent thickened to a grease consistency with about 5-25 by weight of lithium soaps of soap-forming substantially saturated hydroxy fatty material, (LS-5% by weight of a calcium salt of mahogany sulfonic acids and about (Ll-1% by weight of a water-soluble corrosion inhibitor of the group consisting of alkali metal nitrites and alkali metal chromates.

9. A lubricating grease comprising a mineral lubricating oil as the predominant constituent thickened to a grease consistency with about 525% by weight of lithium soaps of soap-forming substantially saturated hydroxy fatty material, (LS-5% by'weight; of an oil soluble calcium salt of hydrocarbyl sulfonic acids, said acids having molecular weights from about 400 to about 750, and about (Ll-1% by weight of a water-soluble corrosion inhibitor of the group consisting of alkali metal nitrites and alkali metal chromates.

Duncan et a1. Dec. 7, 1948 Zimmer et a1. Oct. 4, 1949 

1. A LUBRICATING COMPOSITION COMPRISING A MINERAL LUBRICATING OIL AS THE PREDOMINANT CONSTITUENT, THE OIL BEING THICKENED TO A GREASE CONSISTENCY WITH LITHIUM 12HYDROXY STEARATE, SAID GREASE ALSO COINTAINING ABOUT 1.5% CALCIUM MAHOGANY SULFONATES, 0.5% SODIUM NITRITE, 0.6% WATER AND 1% PHENYL-A-NAPHTHYLAMINE. 